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**Analogue Electronics II EMT 212/4**

Chapter 1 Operational Amplifier Semester /2011

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**1.0 Operational Amplifier**

1.1 Introduction 1.2 Ideal Op-Amp 1.3 Op-amp Input Modes 1.4 Op-amp Parameters 1.5 Operation Single-mode Differential-mode Common-mode operation 1.6 Op-Amps Basics 1.7 Practical Op Amp Circuits 1.8 Op Amp Datasheet

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**1.1 Introduction Typical IC packages**

IC packages placed on circuit board

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**1.1 Introduction Definition**

The operational amplifier or op-amp is a circuit of components integrated into one chip. A typical op-amp is powered by two dc voltages and has one inverting(-) input, one non-inverting input (+) and one output. Op-amps are used to model the basic mathematical operations ; addition, subtraction, integration and differentiation in electronic analog computers. Other operations include buffering and amplification of DC and AC signals.

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**1.1 Introduction Two Power Supply (PS) +V : Positive PS**

-V : Negative PS One Output Terminal Op-amp schematic symbol Two Input Terminals Inverting input Non-inverting input

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**1.1 Introduction Applications of Op-Amp**

To provide voltage amplitude changes (amplitude and polarity) Comparators Oscillators Filters Sensors Instrumentation amplifiers

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**1.1 Introduction Stages of an op-amp OUTPUT STAGE INPUT STAGE**

GAIN STAGE

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1.1 Introduction Typical op-amp packages

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**1.1 Introduction The 741 op-amp Real op-amp : 741**

Literally a black box

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1.2 Ideal Op-Amp Practical Op-Amp Ideal Op-Amp

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**1.2 Ideal Op-Amp Properties Ideal Op-Amp Practical Op-Amp**

Infinite input impedance Zero output impedance Infinite open-loop gain Infinite bandwidth Zero noise contribution Zero DC output offset Input impedance 500k-2M Output impedance Open-loop gain (20k to 200k) Bandwidth limited (a few kHz) Has noise contribution Non-zero DC output offset

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**1.2 Ideal Op-Amp Infinite Input Impedance**

Input impedance is measured across the input terminals. It is the Thevenin resistance of the internal connection between the two input terminals. Input impedance is the ratio of input voltage to input current. When Zi is infinite, the input current is zero. The op amp will neither supply current to a circuit nor will it accept current from any external circuit. In real op-amp, the impedance is 500k to 2M

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**1.2 Ideal Op-Amp Zero Output Impedance**

Looking back into the output terminal, we see it as a voltage source with an internal resistance. The internal resistance of the op-amp is the output impedance of op-amp This internal resistance is in series with the load, reducing the output voltage available to the load Real op-amps have output impedance in the range of .

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**1.2 Ideal Op-Amp Infinite Open-Loop Gain**

Open-Loop Gain, A is the gain of the op-amp without feedback. In the ideal op-amp, A is infinite In real op-amp, A is 20k to 200k

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**1.2 Ideal Op-Amp Infinite Bandwidth**

The ideal op-amp will amplify all signals from DC to the highest AC frequencies In real op-amps, the bandwidth is rather limited This limitation is specified by the Gain-Bandwidth product, which is equal to the frequency where the amplifier gain becomes unity Some op-amps, such as 741 family, have very limited bandwidth, up to a few kHz only

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**1.2 Ideal Op-Amp Zero Noise Contribution**

in an ideal op amp, all noise voltages produced are external to the op amp. Thus any noise in the output signal must have been in the input signal as well. the ideal op amp contributes nothing extra to the output noise. In real op-amp, there is noise due to the internal circuitry of the op-amp that contributes to the output noise

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**1.2 Ideal Op-Amp Zero Output Offset**

The output offset voltage of any amplifier is the output voltage that exists when it should be zero. The voltage amplifier sees zero input voltage when both inputs are grounded. This connection should produce a zero output voltage. If the output is not zero then there is said to be an output voltage present. In the ideal op amp this offset voltage is zero volts, but in practical op amps the output offset voltage is nonzero (a few miliVolts).

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**1.2 Ideal Op-Amp Both Differential Inputs Stick Together**

this means that a voltage applied to one inverting inputs also appears at the other non-inverting inputs. If we apply a voltage to the inverting input and then connect a voltmeter between the non-inverting input and the power supply common, then the voltmeter will read the same potential on non-inverting as on the inverting input.

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**1.3 Op-Amp Input Modes Single-Ended Input Mode**

Input signal is connected to ONE input and the other input is grounded. Non- Inverting Mode input signal at +ve terminal output same polarity as the applied input signal Inverting Mode input signal at –ve terminal output opposite in phase to the applied input signal

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**1.3 Op-Amp Input Modes Differential Input Mode**

TWO out-of-phase signals are applied with the difference of the two amplified is produced at the output.

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**1.3 Op-Amp Input Modes Common Mode Input**

Two signals of same phase, frequency, and amplitude are applied to the inputs which results in no output (signals cancel). But, in practical, a small output signal will result. This is called common-mode rejection. This type of mode is used for removal of unwanted noise signals.

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**1.4 Op-Amp Parameters COMMON-MODE REJECTION (CMRR)**

COMMON-MODE INPUT VOLTAGE INPUT OFFSET VOLTAGE INPUT BIAS CURRENT INPUT IMPEDANCE INPUT OFFSET CURRENT OUTPUT IMPEDANCE SLEW RATE

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**1.4 Op-Amp Parameters Common-Mode Rejection Ratio (CMRR)**

The ability of amplifier to reject the common-mode signals (unwanted signals) while amplifying the differential signal (desired signal) Ratio of open-loop gain, Aol to common-mode gain, Acm The open-loop gain is a datasheet value The higher the CMRR, the better, in which the open-loop gain is high and common-mode gain is low. CMRR is usually expressed in dB & decreases with frequency

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**1.4 Op-Amp Parameters Common-Mode Input Voltage Input Offset Voltage**

The range of input voltages which, when applied to both inputs, will not cause clipping or other output distortion. Input Offset Voltage Ideally, output of an op-amp is 0 Volt if the input is 0 Volt. Realistically, a small dc voltage will appear at the output when no input voltage is applied. Thus, differential dc voltage is required between the inputs to force the output to zero volts. This is called the Input Offset Voltage, Vos. Range between 2 mV or less.

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**1.4 Op-Amp Parameters Input Bias Current Ideally should be zero**

The dc current required by the inputs of the amplifier to properly operate the first stage. Is the average of both input currents

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**1.4 Op-Amp Parameters Input Impedance**

Is the total resistance between the inverting and non-inverting inputs. Differential input impedance : total resistance between the inverting and non-inverting inputs Common-mode input impedance: total resistance between each input and ground

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**1.4 Op-Amp Parameters Input Offset Current**

Is the difference of input bias currents Input offset current Offset voltage Thus, error

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**1.4 Op-Amp Parameters Output Impedance Ideally should be zero**

Is the resistance viewed from the output terminal of the op-amp. In reality, it is non-zero.

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**1.4 Op-Amp Parameters Slew Rate**

Is the maximum rate of change of the output voltage in response to a step input voltage.

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**1.4 Op-Amp Parameters Slew Rate**

It’s a measure of how fast the output can “follow” the input signal.

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1.4 Op-Amp Parameters Example Determine the slew rate:

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**1.5 Operation Types of Op-amp Operation Differential Amplifier Circuit**

If an input signal is applied to either input with the other input is connected to ground, the operation is referred to as ‘single-ended.’ If two opposite-polarity input signals are applied, the operation is referred to as ‘double-ended.’ If the same input is applied to both inputs, the operation is called ‘common-mode.’

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1.5 Operation Differential Amplifier Circuit Basic amplifier circuit

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**1.5 Operation Differential Amplifier Circuit DC ANALYSIS**

DC bias of differential amplifier circuit DC ANALYSIS

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**1.5 Operation Example : Differential Amplifier Circuits**

Calculate the dc voltages and currents

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1.5 Operation Differential Amplifier Circuit Example Solution

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**1.5 Operation Differential Amplifier Circuit AC ANALYSIS Single-Ended**

Connection to calculate : Av1 = Vo1 / Vi1

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**1.5 Operation Differential Amplifier Circuit AC ANALYSIS Single-Ended**

B E C AC equivalent of differential amplifier circuit

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**1.5 Operation Differential Amplifier Circuit KVL**

AC Analysis Single ended Scan figure & 10.15 Partial circuit for calculating Ib

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**1.5 Operation Differential Amplifier Circuit Example Solution**

Calculate the single-ended output voltage Vo1

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**1.5 Operation Differential Amplifier Circuit**

AC Analysis Double ended A similar analysis can be used to show that for the condition of signals applied to both inputs, the differential voltage gain magnitude is

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**1.5 Operation Differential Amplifier Circuit AC Analysis - Common-mode**

Common-mode connection

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1.5 Operation Differential Amplifier Circuit AC Analysis - Common-mode

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